Friction materials including chrysotile asbestos fibers therein have been conventionally used for brake devices. However, the friction material including chrysotile asbestos is low in thermal stability and rapidly decreases in coefficient of friction at relatively low temperatures to undergo fading, while the friction material markedly wears away at high temperatures. Thus it has been desired to alleviate-these problems. Furthermore, there is a tendency to control the use of asbestos because of its carcinogenicity from the viewpoint of environmental hygiene, so that it is strongly demanded to develop substitutes for chrysotile asbestos fibers.
To meet the demand, attempts have been made to use in various fields for engineering applications potassium titanate fibers which are represented by the general formula K.sub.2 O.nTiO.sub.2 and typical of which are potassium hexatitanate fiber (K.sub.2 Ti.sub.6 O.sub.13), potassium tetratitanate fiber (K.sub.2 Ti.sub.4 O.sub.9) and potassium octatitanate fiber (K.sub.2 Ti.sub.8 O.sub.17), as substitutes for asbestos fibers. Among these, potassium hexatitanate fiber is effective for the prevention of fading because of its high heat resistance, has Mohs' hardness of about 4, is therefore less likely to abrade the adjoining materials, and is useful, for example, for preventing abnormal actuation of brakes since it has less hygroscopicity and is not reactive with water. By virtue of these properties, this fiber was proposed to use for friction materials for automotive brake pads, etc. (Unexamined Japanese Patent Publications SHO 61-191599 and HEI 1-294553).
However, it has been found that although the friction material including the potassium hexatitanate fibers has increased resistance to wear, the coefficient of friction thereof at low temperatures (up to about 250.degree. C.) is rather lower than that of the friction material including the chrysotile asbestos fibers. With an increase in the speed of vehicles, a higher coefficient of friction is required of the friction material for brake devices in recent years, and it is desired to provide friction materials retaining a high coefficient of friction over a low to high, wide temperature range.
The present invention provides a fiber material satisfying the above demand, that is, a polycrystalline fiber having a composite phase or structure which consists of potassium hexatitanate crystals and titanium dioxide crystals as mixed therewith.
Incidentally, rutile-priderite-potassium hexatitanate composite fiber is known as a polycrystalline fiber having a composite structure which comprises potassium hexatitanate crystals and other crystals (Unexamined Japanese Patent Publications SHO 60-34617 and SHO 60-259627), whereas this composite fiber differs from the composite fiber of the present invention in that it contains priderite. The composite fiber containing priderite not only exhibits lower strength but also has the problem of becoming brown in its entirety.